For example, when an input voltage of a drive equipment such as an inverter changed according to the input power source fluctuation, an output of PWM converter may generally result in overcurrent or overvoltage. This is because PLL control of a converter output voltage cannot be followed to the input power source fluctuation.
In a PWM converter, when PLL control becomes unusual, the converter output and the inverter output are used to suspend with the gating signal (hereinafter, a gate block is called) like a case when general momentary power failure occurred. And when the input power source fluctuation is eliminated, the converter and the inverter are restarted. Proposals have been made to prevent a DC overvoltage and to continue an operation of the converter. (For example, refer to Patent documents 1).